An optical memory technology using an optical disc having a pit-like pattern as a high-density and large-capacity storage medium has been practically used, while expanding its application to a digital audio disc, a video disc, and a document file disc and further to a data file. It is desired that the function, which is roughly sub-divided into a light focusing function which forms a minute spot of a diffraction limited size, a focus control (focus servo) function for an optical system, a tracking control function, and a pit signal (information signal) detecting function, is successfully performed with high reliability.
In recent years, due to the advancement of an optical system design technology and the achievement of a shorter wavelength of a semiconductor laser as a light source, the development of an optical disc having a storage capacity at a higher-than-ever density has advanced. As an approach to achieve a higher density, there is a method for increasing an optical-disc-side numerical aperture (NA) in a focusing optical system which converges a light beam to a minute spot on an optical disc. At this time, an increase in the amount of aberration produced by the inclination (so-called tilt) of an optical axis presents a problem. When the numerical aperture NA is increased, the amount of aberration produced by the tilt increases. To prevent this, it is appropriate to reduce the thickness of the substrate (base member thickness) of the optical disc.
In a compact disc (CD) which might be said to be a first generation optical disc, infrared light having a wavelength λ3 (the wavelength λ3 is 780 nm to 820 nm) and an objective lens having a numerical aperture NA of 0.45 are used, and the base member thickness of the optical disc is 1.2 mm. In a second generation DVD, red light having a wavelength λ2 (the wavelength λ2 is 630 nm to 680 nm, and a standard wavelength is 650 nm) and an objective lens having a numerical aperture NA of 0.6 is used, and the base member thickness of the optical disc is 0.6 mm. In a third generation optical disc, blue light having a wavelength λ1 (the wavelength λ1 is 390 nm to 415 nm, and a standard wavelength is 405 nm) and an objective lens having a numerical aperture NA of 0.85 is used, and the base member thickness of the optical disc is 0.1 mm. Note that, in the present specification, a thickness of a substrate (base member thickness) refers to a thickness from a surface of an optical disc (or optical information medium) on which a light beam is incident to a recording layer thereof on which information is recorded. Thus, the thickness of the substrate of the high-density optical disc has been reduced compared with the related art optical disc.
As another method for increasing the storage capacity of an optical disc, the number of recording layers is increased. Between the recording layers, an intermediate layer needs to be provided so as to prevent the occurrence of leak-in of information. However, a spherical aberration generated when the thickness from the top surface of the optical disc to the recording layer thereof changes from an expected value is proportional to approximately the fourth power of the numerical aperture. Therefore, when the numerical aperture is set high, it is undesirable to design thicken the intermediate layer. As a result of reducing the thickness of the intermediate layer, the leak-in of information (crosstalk) between the recording layers and interference by reflected light from each of the recording layers present a problem. One of countermeasures against the problem is disclosed in Patent Literature 1. Using FIG. 20, Patent Literature 1 will be briefly described as a related-art example.
FIG. 20 is a view showing a schematic configuration of an optical head of the related-art example. FIG. 21 is a view showing a schematic configuration of an optical disc of the related-art example. FIG. 22 is a view showing a schematic configuration of a detection hologram of the related-art example.
The optical head 500 includes a light source 501 which emits blue-violet laser light, a beam splitter 502, a collimator lens 503, an objective lens 504, a detection hologram 505, a detection lens 506, and a light receiving element 507 which receives laser light. An optical disc 508 includes three information recording layers. The optical head 500 not only records or reproduces information by causing the blue-violet laser light to pass through two types of base members having different thicknesses, but also records or reproduces information on or from the optical disc 508 having the plurality of information recording layers.
Using FIG. 20, a description will be given to an operation of the optical head 500 which records or reproduces information on or from the optical disc 508. The blue-violet laser light emitted from the light source 501 is transmitted by the beam splitter 502 and converted by the collimator lens 503 into generally parallel light to be incident on the objective lens 504. Then, the blue-violet laser light incident on the objective lens 504 is converged to a light spot onto any of the information recording layers of the optical disc 508 through a protective substrate.
The blue-violet laser light in a return path reflected by the information recording layer of the optical disc 508 follows the same optical path as followed in an outward path and is transmitted by the objective lens 504 and the collimator lens 503. The blue-violet laser light transmitted by the collimator lens 503 is reflected by the beam splitter 502, then divided by the detection hologram 505 for the detection of a servo signal, imparted with a predetermined astigmatism by the detection lens 506, and guided to the light receiving element 507. As a result, an information signal and the servo signal are generated.
A focus error signal for the optical disc 508 is generated using a so-called astigmatic method in which a focal spot imparted with an astigmatism by the detection lens 506 is detected with a quartered light receiving pattern in the light receiving element 507 or the like. A tracking error signal for the optical disc 508 is generated using a zero-order diffracted light beam and plus first-order diffracted light beams each generated by the detection hologram 505. The numerical aperture (NA) of the objective lens 504 is 0.85. The objective lens 504 is designed to be capable of forming a focal spot of a diffraction limited size onto any of the information recording layers provided in the optical disc 508 in which the thickness of a protective layer is about 0.1 mm.
As shown in FIG. 21, the optical disc 508 includes first to third information recording layers 511, 512, and 513 in which protective layers have mutually different thicknesses. Accordingly, when the focal spot is formed on, e.g., the second information recording layer 512 and information is recorded or reproduced on or from the second information recording layer 512, laser light is reflected also by each of the first and third information recording layers 511 and 513. The laser light beams are transmitted by the objective lens 504 and the collimator lens 503, and reflected by the beam splitter 502, similarly to the laser light (signal light) beam reflected by the second information recording layer 512. Then, the laser light beams reflected by the beam splitter 502 are transmitted by the detection hologram 505 and the detection lens 506, and guided to the light receiving element 507. The laser light beams reflected by the first and third information recording layers 511 and 513 other than the second information recording layer 512 on which the focal spot is formed and incident on the light receiving element 507 are called so-called stray light beams.
The detection hologram 505 has a light blocking region 505x as shown in FIG. 22. The light blocking region 505x is a circular region having a diameter D2. The light blocking region 505x is formed by, e.g., vapor depositing a metal film of aluminum or the like. The transmissivity of the light blocking region 505x is substantially zero.
FIG. 23 is a view schematically showing the optical path of the reflected light resulting from reflection by the first information recording layer 511 located on a side closer to a laser light incident surface, when information is recorded or reproduced on or from the second information recording layer 512 of the optical disc 508 using the optical head 500 of the related-art example. The laser light reflected by the first information recording layer 511 has the middle portion thereof blocked by the light blocking region 505x formed in the detection hologram 505 to be transmitted by the detection lens 506 and guided to the light receiving element 507. The laser light reflected from the first information recording layer 511 has light (light in the center portion thereof) including the optical axis of the laser light which is blocked by the light blocking region 505x, and does not enter a light receiving portion in the light receiving element 507.
FIG. 24 is a view schematically showing the optical path of the reflected light resulting from reflection by the third information recording layer 513 located on a side more distant from a laser light incident surface, when information is recorded or reproduced on or from the second information recording layer 512 of the optical disc 508 using the optical head 500 of the related-art example. The laser light reflected by the third information recording layer 513 also has light (light in the center portion thereof) including the optical axis of the laser light which is blocked by the light blocking region 505x, and does not enter the light receiving portion in the light receiving element 507.
As described above, the laser light (another layer reflected light) beams reflected by the first information recording layer 511 and the third information recording layer 513 are prevented by the light blocking region 505x from entering the light receiving portions in the light receiving element 507. Therefore, the laser light beams reflected by the first information recording layer 511 and the third information recording layer 513 do not overlap the laser light beam reflected by the second information recording layer 512 as the target of information recording or reproduction. As a result, fluctuations in the amount of the detected laser light beam reflected by the second information recording layer 512 are suppressed, and stabilization of the servo signal and the information signal can be achieved.
The related art example discloses the light blocking region including the optical axis as a means for avoiding leak-in of information (crosstalk) between the individual information recording layers and interference between the reflected light beams from the individual information recording layers in the case of further increasing the number of multiple layers in an ultra-high-density optical disc. However, to prevent the laser light beams reflected by the first information recording layer 511 and the third information recording layer 513 from entering the light receiving portions, the area of the light blocking region should be increased or the magnification of a signal detection optical system should be increased.
However, as the area of the light blocking region is increased, the blocked range of the signal light beam for obtaining the information signal increases to undesirably reduce the amount of the signal light beam. The magnification of the signal detection optical system mentioned above is the ratio of a focal distance fd for focusing the laser light beam toward the light receiving element (or photodetector) to a focal distance f0 for focusing the laser light beam toward the optical disc by means of the objective lens, i.e., fd/f0. To increase the ratio fd/f0, the focal distance fd may be increased appropriately but, when the foal distance fd is increased, the problem of an increased device size occurs.
The ratio fd/f0 can also be increased by reducing the focal distance f0, but the effective diameter of the objective lens is also reduced. Here, the objective lens needs to be moved in a direction perpendicular to the optical axis so as to follow the eccentricity of the optical disc. However, when the effective diameter of the objective lens is reduced, the problem arises that the amount of movement of the map of the effective diameter of the objective lens over the light receiving element increases to degrade a control signal. In either case, it is not preferable to increase the ratio fd/f0.
FIG. 25 is a view schematically showing a light spot formed on the light receiving portions of the related art optical head. As a measure to reduce the problem described above, a method can be considered in which, as shown in FIG. 25, an another layer light beam 320 which is a reflected light beam from another information recording layer is not prevented from entering light receiving portions 309a to 309d, but the another layer light beam 320 is incident on the light receiving portions 309a to 309d and blocked so as not to overlap a signal light beam 321. This allows the magnification fd/f0 of the signal detection optical system to be reduced to a value lower than in an optical head which satisfies the condition that reflected light from another information recording layer does not enter a light receiving region. Note that a reflected light beam from an information recording layer other than the information recording layer on which a laser light beam is focused to a light spot for the reproduction or recording of information is called an “another layer light beam”.
However, the problem reducing measure has another problem. FIG. 26 is a view showing reflected light beams from three information recording layers included in an optical disc when a laser light beam is focused on the third information recording layer. FIG. 27 is a view schematically showing the light spot of each of reflected light beams from the second and third information recording layers, which is formed on the light receiving portions when the laser light beam is focused on the third information recording layer shown in FIG. 26. FIG. 28 is a view schematically showing the light spot of each of reflected light beams from the first and third information recording layers, which is formed on the light receiving portions when the laser light beam is focused on the third information recording layer shown in FIG. 26.
For example, when the laser light is focused on the third information recording layer 513 in FIG. 26, reflected light beams are also generated from the second information recording layer 512 and the first information recording layer 511. As shown in FIG. 27, an another layer light beam 420 reflected by the second information recording layer 512 has light in the vicinity of the optical axis thereof which is blocked and therefore does not overlap a signal light beam 421 reflected by the third information recording layer 513. This is similar to the another layer light beam 320 from another information recording layer shown in FIG. 25.
As also shown in FIG. 28, an another layer light beam 520 reflected by the first information recording layer 511 expands to be larger over the light receiving surface of the light receiving element since the distance between the third information recording layer 513 and the first information recording layer 511 is larger than the distance between the third information recording layer 513 and the second information recording layer 512. As a result, the another layer light beam 520 reflected by the first information recording layer 511 does not overlap the signal light beam 421 reflected by the third information recording layer 513. However, there remains the possibility of a new problem that the another layer light beam 420 reflected by the second information recording layer 512 and the another layer light beam 520 reflected by the first information recording layer 511 overlap to interfere with each other. The mechanism of the interference will be described using FIG. 29.
FIG. 29 is a view schematically showing the light spot of each of the reflected light beams from the first to third information recording layers, which is formed on the light receiving portions when the laser light beam is focused on the third information recording layer shown in FIG. 26.
As described previously, the provision of the light blocking region in the vicinity of the optical axis can reduce mutual interference between the signal light beam 421 and the another layer light beams 420 and 520. In addition, in a region 430 (horizontally hatched portion) where only the another layer light beam 420 reflected by the second information recording layer 512 is present, the another layer light beam 520 is not present so that interference between the another layer light beam 520 and the another layer light beam 420 does not occur. What presents a problem is a region (portion hatched with left downwardly inclined lines) outside the region 430. In the region outside the region 430, the another layer light beam 420 reflected by the second information recording layer 512 and the another layer light beam 520 reflected by the first information recording layer 511 overlap so that interference occurs therebetween. If the another layer light beam 420 and the another layer light beam 520 overlap on the light receiving portions 309a, 309b, 309c, and 309d, when a change in the spacing between the first information recording layer 511 and the second information recording layer 512 or the like causes a fluctuation in the state of the interference, the signal light beam 421 may be affected by the interference to deteriorate and degrade the S/N ratio of the signal outputted from the light receiving portions.
This problem may possibly occur when an optical information medium on or from which an optical head is to reproduce or record information has three or more information recording layers. Preferably, particular attention is given to the case where there are two or more information recording layers on at least one of the side of the information recording layer on which the laser light beam is focused which is closer to the laser light incident surface and the side thereof more distant from the laser light incident surface.
If consideration is given to the case where the laser light is focused on the third information recording layer 513 and the another layer light beams are generated from the second information recording layer 512 and the first information recording layer 511, the phases of the wavefronts of the another layer light beam from the second information recording layer 512 and the first information recording layer 511 are close to each other. As a result, the intensity difference between the interference fringes of the two another layer light beams becomes relatively large to exert particularly large influence when the interfering light beams are incident on the light receiving portions.